n. The spine.n. n. The continued stem or midrib of a pinnately compound leaf, as in a rose leaf or a fern.n. The principal axis in a raceme, spike, panicle, or corymb.n. n. The shaft of a feather. The rhachis of the after-shaft, or plumule, is called the hyporhachis.n. The central cord in the stem of a crinoid.n. The median part of the radula of a mollusk.n. A central cord of the ovary of nematodes.